Home Digital Systems
Part Eleven – Test Equipment for TV and Satellite Reception R A Calaz C Eng, B Sc(Eng), MIET, ACGI, MSCTE Copyright Notice All rights reserved. No part of this publication may be reproduced without the express written permission of the author. All logos and trademarks are the copyright of their respective owners. The right of R A Calaz to be identified as the author of this work has been asserted by him in accordance with the Copyright Designs and Patents Act 1998. Disclaimer This publication is intended to provide information regarding the installation of digital systems in a residential environment. Every effort has been made to make it as complete and accurate as possible, but no warranty of fitness is implied. The author shall have no responsibility with respect to loss or damage arising from the information contained in this document. Corrections and comments should be addressed to [email protected].
About the Author Bob Calaz is an acknowledged expert in the field of TV, satellite and multimedia installations having been a chartered electrical engineer for more than 40 years. For 26 of those years, he was with the Rediffusion group of companies, including a secondment to South Africa for 15 years as chief engineer of Rediffusion South Africa. He was a member of a government committee set up to define the technical standards for TV systems in multi-dwelling units, and was responsible for the design and installation of a 200 channel TV distribution system for the headquarters of the South African Broadcasting Corporation. Returning home to the UK in 1985, he founded Race Communications Ltd (based in Berkshire) to service the growing TV industry. He has since conducted many technical training courses for commercial and military personnel. Bob has presented numerous technical articles and papers throughout the world and has been regularly in demand as an advisor to a variety of commercial, industry and governmental bodies. His recent publications include two reference books on Digital TV, Satellite and Multimedia.
The complete “Insider Guide” set of publications This publication is one of a series covering the design, installation, operation and maintenance of digital reception and distribution systems in the home. The complete series is sub-divided as follows: This publication is one of a series covering the design, installation, operation and maintenance of digital reception and distribution systems in the home. The complete series is sub-divided as follows: Part 1 - Fundamentals of Electricity Part 2 - Digital Television Part 3 - Digital TV displays Part 4 - TV Modulation Techniques Part 5 - UHF TV Broadcasting and Reception Part 6 - Radio and TV Aerial Installations Part 7 - UHF TV Signal Distribution Part 8 - Satellite TV Reception Part 9 - Satellite TV Distribution Part 10 - Satellite IF Network Planning Part 11 - Test Equipment Part 12 - Fibre Optic Distribution Part 13 - Distribution of Voice and Data Signals Part 14 - Digital Home Technologies Part 15 - Structured Cable Networks Part 16 - VSAT Systems Part 17 - Abbreviations/Glossary of Terms
Introduction This series of publications introduces the reader to the application of digital systems in the home. It is suitable for those with no prior knowledge of the subject, whilst at the same time providing a source of reference for experienced installers who wish to know more about the subject. This part (part 11) specifies the terrestrial UHF and satellite Ku frequency bands, and the various measurement parameters for TV signals. Constellation diagrams are also described. The most suitable measurement location in the reception chain is determined. The various types of meter are then described together with their measurement parameters. Spectrum analysers are then introduced, with descriptions of how the displays can be used to investigate and measure the quality of signals. Parts one to twelve cover radio, TV and satellite reception and distribution; parts thirteen to sixteen cover data systems and part seventeen is a reference of abbreviations and a glossary of terms.
Book 11 Test Equipment for TV and Satellite Reception 1 The Frequency Spectrum 2 Measurement Parameters 3 Signal Level Meters 4 Spectrum Analysers
1. The Frequency Spectrum The procedure for measuring signal parameters consists of two steps: Select the required range of frequencies over which the measurements are to be carried out. Perform the required measurements on the selected frequency range. The relevant frequency ranges are as follows: UHF DTT 470 – 800MHz Satellite IF 950 – 2150MHz The UHF terrestrial frequency spectrum is divided into channels 21-60, each 8MHz wide. Each transmitter location broadcasts up to six digital multiplexes – the channel allocations for the Crystal Palace channels are shown below: Other multiplexes are also being broadcast on a temporary basis. The digital information is contained in many carrier frequencies spread across nearly all of the 8MHz channel bandwidth and it is necessary to measure the average power of all these carrier frequencies together. The measuring device is normally set to the centre frequency of the 8MHz channel
although most meters do this automatically when the channel number is selected. Basic meters measure the centre frequency and interpolate the power of the whole multiplex, whereas the more sophisticated versions measure the power of all the carriers and integrate them together: Locally generated analogue UHF channels have the same 8MHz bandwidth, and the amplitude of the vision carrier is measured – in the UK, this is1,25MHz above the start frequency of the channel. The next drawing shows the components of an analogue UHF channel. Analogue signals are normally transmitted up to 20dB higher than the levels of the digital multiplexes: Satellite IF digital signal levels are measured in the same way. Some meters have an adjustable bandwidth and this should be set to the bandwidth of the signal being measured (normally 30MHz or 40MHz). The meter should then be tuned to the centre frequency of the multiplex and the digital signal levels recorded.
It is important to examine the signal levels, not just for one channel, but for all channels across the frequency band. Cable losses increase with frequency so the signal levels could be less on the higher channels. Some channels may have extraneous interference from sources such as microwave towers, radar and TETRA relay sites, all of which may degrade the signals. It is also advisable to check the levels of signals in other frequency bands, to ensure that they will not overload any amplifiers in the system:
2. Measurement Parameters Signal Levels The signal levels into a DTT or satellite receiver must be within the following windows of operation: DTT 50-80dBµV Satellite 47-77dBµV Too low a signal will result in degradation from noise, and too high a signal will overload the input circuitry. Carrier to Noise Ratio As described in book 4, data is relayed by varying the phase and amplitude of each cycle of a carrier wave. The rotating vector represents the “1” and “0” data bits in sequence. This 16QAM drawing shows sixteen different “states” per cycle: If each of the 16 states is a single dot, this indicates a clean (and therefore robust) digital signal. Noise or distortion on a data symbol will cause changes to the amplitude and/or phase of the rotating vector:
If the dots are dispersed, the symbol is more likely to be wrongly interpreted, causing errors: As long as each “hit” stays within the amplitude/phase boundary representing a particular binary number, the receiver will interpret the data correctly: If the distortion becomes more severe, the dots spread out, until, eventually, they fall into an adjacent boundary - the digital receiver will then interpret the information wrongly, causing an error. The greater the noise, the more likely it is that this will occur. These drawings are known as “constellation diagrams” which can be displayed on some makes of spectrum analyser. They provide a very useful indication of the robustness of a data signal. It is therefore most important to measure the amplitude of any noise induced on to the received signal, in relation to the amplitude of the carrier wave – this is known as the Carrier-to-Noise ratio (C/N).
The higher the C/N ratio, the better. The minimum C/N ratio required depends on the type of modulation used and the amount of error correction applied to the received signal, as described earlier in this document. For terrestrial DTT signals using 64QAM (FEC 2/3), the minimum C/N ratios are as follows: Pass ≥26dB Marginal 22 – 26dB Fail ≤22dB Ideally, the noise level should be measured in the same UHF channel as the received signal. Since this is clearly not possible, the noise measurement should be made at a frequency as close to the UHF channel as possible and where no other UHF signal is present – this is known as a “near noise” measurement. If there are occupied channels on either side of the one being measured, it will be necessary to make a “far noise” measurement on a nearby unoccupied UHF channel: Some spectrum analysers allow the user to define the frequency at which the noise measurement is made. This should be as close as possible to the channel being evaluated, since the noise level can vary over the UHF frequency band.
The same comments apply to satellite signals. The minimum C/N ratio depends on the amount of error correction being applied, typical values being as follows: Astra @ 28 oE (FEC = 2/3) 10dB Eutelsat @ 13 oE (FEC = ¾) 12dB Bit Error Ratio (BER) BER measurements can be made at various points in the reception chain: Before any error correction is applied – this is known as the “channel BER” or CBER. After Viterbi and before Reed Solomon error correction – called the “post- Viterbi BER” or VBER. After all the error correction has been applied (LBER). At this point, there should of course be no errors at all. Some spectrum analysers can display the BER at all these locations, and even provide a running count of how many errors have occurred at the MPEG2 video decoder over the entire time period since the analyser locked on to the incoming data stream. BER measurements are normally made “post-Viterbi” and “pre-Reed Solomon”, where the maximum permitted number of errors for both DTT and satellite IF measurements is 2 errors in every 10 000 bits of data received – this will result in fewer than one noncorrectable error for every transmission hour, which is known as Quasi-Error Free “QEF”. The amount of Viterbi correction depends on the FEC applied by the broadcaster, but it can be very substantial. BER measurements cannot easily be made post-Reed Solomon because the errors occur so infrequently. The BER is normally quoted in the form X.X E - X (scientific notation) eg 1E-4 This can be interpreted as 1 part in 1 x 10 4 = 1 part in 10 000. Similarly, 1E-7 = 1 part in 1x 10 7 = 1 part in 10 000 000. Constellation Diagrams The following pictures indicate how to analyse DTT constellation diagrams: This picture shows a good 64QAM signal with a 30dB C/N ratio. The smaller the dots, the better the C/N ratio:
This is what happens when the signal does not lock. The receiver is unable to recognise the data being received: Here, the constellation has locked, but the C/N ratio is very marginal: Phase noise is visible on this display. This is a circular effect that is most pronounced at the perimeter. Phase noise is most likely due to the head-end equipment and is not caused by normal transmission along a network:
This has been named the “polo mint” effect. It is caused by an “in-channel” spurious analogue signal approximately 25dB less than the QAM signal. The digital picture would still be perfect but the offending signal should be traced before it causes trouble: This display shows a DTT channel from the Crystal Palace transmitter: The digital picture quality on a TV receiver is perfect, but the system is about to fail because the C/N ratio is only 19dB. The constellation diagram clearly shows that there is a problem that must be solved! Modulation Error Ratio (MER)
This is the power of the carrier level, compared to the power of the noise present in the constellation. In effect, it is the C/N ratio with the noise measured within the multiplex. Some meters will measure this for specified individual carriers within a COFDM multiplex. The minimum MER values depend on the amount of error correction applied and are as follows: DTT – FEC Limit Satellite IF – FEC Limit 2/3 25dB 2/3 9dB 3/4 20dB 3/4 12dB 5/6 13dB Some meters also indicate the noise margin. This indicates the safety margin in dB above the MER limit at which the BER would be at its recommended minimum value.
3 Signal Level Meters Terrestrial Many installers still use a tunable signal level meter for the alignment of terrestrial aerials. The measurement procedure is as follows: Tune the meter to the required channel or multiplex. Adjust the fixed attenuators to suit the signal level. Adjust the frequency to peak the reading. Measure and record the signal level in dB V or dBmV. Terrestrial measurements are usually taken for each channel that is required. It is usually best to peak the aerial on the weakest channel so as to obtain equal signal strength on all channels, rather than the maximum signal on the strongest channels. Digital measurements should also include C/N ratio and BER. This meter displays the levels of a number of channels simultaneously. For digital measurements, it also gives an indication of BER by indication a Pass (P) or Fail (F) for each multiplex. Satellite The procedure is much the same. Satellite measurements should be taken on sample multiplexes at the bottom, middle and top of each of the four satellite IF frequency bands.
This meter also has a spectrum display.
4 Spectrum Analysers A spectrum analyser displays the instantaneous amplitude of an incoming signal over a specified range of frequencies: Analysers are battery-powered, mostly with rechargeable batteries. The display device used to be a cathode ray tube (CRT) although modern versions tend to use an LCD display which is lighter, less bulky and consumes less power. Versions are available for terrestrial or satellite applications, or both. Most modern spectrum analysers are sophisticated instruments that can provide a wealth of information not previously available, to assist the installer and make his job a lot easier. A more detailed schematic layout of such a unit is shown below: The basic functions of a spectrum analyser are as follows: Measurement of RF parameters such as channel power (with an adjustable bandwidth) and C/N ratio. Spectrum display, to detect interfering signals and measure the flatness of the multiplex. A
common interfering signal on satellite IF systems is from DECT phones at 1890MHz. Demodulation of the digital signal (this needs either a COFDM or QPSK demodulator card for terrestrial and satellite applications respectively) in order to measure the bit error ratio (both before and after “Viterbi” error correction has been applied), CSI display and give the quantity of “wrong packets” received. Digital picture display - this needs an MPEG2 decoder card, possibly with a card reader (if encrypted programmes are to be displayed). Some models are offered in a basic format with an upgrade ability to add additional functions by plugging in extra modules. Modern analysers also incorporate some or all of the following additional facilities: Audible indication of signal level (tone with variable pitch). Signal search. External video and audio input and output.. Memory locations for preset measurement parameters. Data storage with a printer port to provide a written record of measurements taken. A white noise generator is a useful accessory. White noise is a flat spectrum of random noise that extends from dc up to very high frequencies. When injected into an amplifier, the output spectrum display will then show the frequency response of the equipment across the frequency band. This is particularly important when aligning frequency conscious equipment such as a filter leveler. Some terrestrial meters and analysers can generate a voltage to power a masthead preamplifier. Satellite versions can have 12/18V switching, 22KHz tone and an integral DiSEqC generator. An important parameter to consider when choosing a spectrum analyser is its dynamic range. This is the range of signal levels that can be resolved on the display screen. A wide dynamic range allows the user to see both strong carrier signals and much weaker noise or interfering signals on the screen at the same time. Spectrum Displays A spectrum analyser is able to display the whole of the frequency spectrum of interest and to expand the display around a selected frequency. This gives some or all of the following specific advantages over a simple level meter: The ability to see and identify interfering or unwanted signals elsewhere in the band that would otherwise not be noticed. The ability to select the frequency at which the noise level is to be measured. The ability to see and measure distortions across a digital multiplex.
This example illustrates the variation of signal level across a DTT multiplex, the higher frequency signals having a greater amplitude – this is called positive slope. This could be caused by the wrong alignment of a channel amplifier or because it is being operated at more than its maximum output. A dip in the centre of the multiplex could be due to standing waves on the network. On satellite IF networks, the ability to see signals of the opposite polarity and minimise them by adjusting the LNB skew. This meter has an “explore” function which scans the selected frequency band, identifies each channel or multiplex found, and stores them for subsequent analysis. It also incorporates a data logger to record the signal parameters achieved for subsequent recording and archive purposes. The IF spectrum displays for Astra @ 28 oE are shown below:
Some satellites occasionally carry both analogue and digital transponders. The analogue signals are easily recognised as shown below:.. It is important to ensure that the amplitude of the multiplexes is reasonably constant across the whole IF band. If the amplitude gets smaller at the higher frequencies, this could result in missing channels, possibly caused by water in the cables or wrong adjustment of the slope control (if fitted): This display shows how an interfering signal can cause loss of reception of the channels in the multiplex on which it is superimposed: